1. Field of the Invention
The present invention relates to an exhaust gas purification device for purifying the exhaust gas of an engine, and more specifically to an exhaust gas purification device having a NOx catalyst for reducing and purifying NOx contained in exhaust gas by using ammonia as a reducing agent.
2. Description of the Related Art
Exhaust gas purification devices for purifying NOx (nitrogen oxide) that is one of the pollutants contained in the exhaust gas discharged from an engine have been well known. To be more concrete, one of such exhaust gas purification devices is provided with a NOx catalyst of an ammonia selective reduction type in the exhaust path of the engine, and purifies the NOx contained in exhaust gas by supplying the NOx catalyst with ammonia serving as a reducing agent.
In this type of exhaust gas purification device, urea water is supplied to the upstream side of the NOx catalyst, and the ammonia created by the urea water being hydrolyzed due to exhaust heat is supplied to the NOx catalyst. The ammonia supplied to the NOx catalyst is adsorbed on the NOx catalyst for a while. A denitrifying reaction caused between the ammonia and the NOx contained in the exhaust gas is advanced by the NOx catalyst, and the NOx is thus purified.
For example, Unexamined Japanese Patent Publication No. 2003-343241 (hereinafter, referred to as Document 1) discloses an exhaust gas purification device of an engine having a NOx catalyst for purifying the NOx contained in exhaust gas by using ammonia as a reducing agent in the above-mentioned manner.
In order to efficiently purify NOx by using the NOx catalyst, it is required that a sufficient amount of ammonia should be adsorbed on the NOx catalyst.
The adsorbable amount of ammonia on the NOx catalyst tends to change in the decreasing direction as the temperature of the NOx catalyst increases. The ammonia adsorbed on the NOx catalyst is liable to desorb from the NOx catalyst along with the temperature rise. If the increase of the NOx catalyst temperature is relatively slow, the ammonia that has desorbed from the NOx catalyst is consumed as a reducing agent for reducing NOx. In contrast, if the NOx catalyst temperature is sharply raised, for example, by rapid acceleration of the engine, the ammonia that has desorbed from the NOx catalyst is also rapidly increased in amount. As a result, part of the ammonia flows out of the NOx catalyst together with the ammonia that has not been adsorbed on the NOx catalyst because of the temperature rise. This increases the amount of ammonia slip.
Therefore, if a sufficient amount of ammonia is adsorbed on the NOx catalyst for the purpose of efficiently purifying NOx, this incurs a large quantity of ammonia slip when the NOx catalyst temperature rapidly climbs. On the other hand, if the amount of ammonia to be supplied to the NOx catalyst is reduced to suppress such ammonia slip, this causes the problem that NOx cannot be efficiently purified.
The exhaust gas purification device disclosed in Document 1 stops the supply of urea water when the exhaust temperature drops. However, this does not prevent the ammonia slip that is increased as the NOx catalyst temperature rises, and would not be a solution for the problem.
Concerning the NOx catalyst using ammonia as a reducing agent, it is desirable that the NOx catalyst be supplied with ammonia of an amount corresponding to the amount of NOx which can be purified by the NOx catalyst, and that all the ammonia be used up for NOx purification. In fact, however, part of the supplied ammonia flows out of the NOx catalyst without contributing to NOx purification.
It is known that an oxidation catalyst is disposed downstream of the NOx catalyst for the purpose of preventing the ammonia that has flowed out of the NOx catalyst from escaping into the atmosphere.
The oxidation catalyst oxidizes the ammonia that has flowed out of the NOx catalyst, and transforms it into N2 or NOx. The NOx produced here changes into N2 due to the ammonia flowing into the oxidation catalyst. In this manner, the ammonia that has flowed out of the NOx catalyst is transformed into nonhazardous N2 by the oxidation catalyst and is released into the atmosphere.
If such an oxidation catalyst is set downstream of the NOx catalyst, it is possible to prevent the ammonia from escaping into the atmosphere. On the other hand, this requires to secure a space for the oxidation catalyst, for example, in the case of an engine installed in a vehicle. There also arises a problem such as a cost increase because expensive noble metal is used as catalytic material for the oxidation catalyst.
If the efficient NOx purification takes precedence on the premise that ammonia slip is prevented by installing the oxidation catalyst, urea water has to be supplied more than required amount in order to compensate the ammonia lost through ammonia slip that is caused by the rise of the NOx catalyst temperature. For that reason, there is the problem that the cost is increased in proportion to the amount of the urea water additionally supplied.